Gas purity testing device



June 17, 19:47. E. J. WEBER GAS PURITY TESTING DEVICE Filedlay 28, 1943 2 Sheets-Sheet 1 F/af.

Engfieqm fare I" 52?; cZW -BER INVENTOQ BTZluu-l, ATTORNET$ a 4K a if.

J n 21, 1947- E J. WEBER 2.422.604

GAS PU RI TY TESTING DEVICE Filed May 28, 1943 Sheets-Sheet 2 EL- 14d 3 441 cf W555? INVENTOR 316E BY ATTORNET$ Patented June 17, 1947 z'nzacoa OFFICE GAS runrrr 'rns'rmc DEVICE Earl .17. Weber, Cleveland, Ohio, asslgnor' to American Gas Association, 1110., Ohio, a corporation or New York Qlevelaml,

Application May 2s, 1943, Serial No. 488,947

. tion, and forming a part of this application, there ings:

Figure 1 is a vertical sectional view of one embodiment of the invention,

Figure 2 is a fragmentary sectional view of a detail of Figure 1, on a larger scale,

Figure 3 is an electrical diagram of the embodiment shown in Figure 1,

Figure 4 is a, developed view of a detail part,

Figure Sis a vertical sectional broken view of another embodiment of the invention, v

Figure 6 is a vertical sectional view of still another embodiment of the invention,

Figure 7 is an electrical diagram of the embodiment shown in Figure 6, and

Figure 8 is a plan view of the embodiment shown in Figure 6.

Referring first to the embodiment shown in Figure 1, the apparatus here shown comprises a metallic casing H) which in this instance includes two generally cylindrical parts It, l2 joined by cooperating threads l3 and divided into two cells or chambers l4, l by a partition '6 in this instance an insulating disc clamped between a. shoulder H on the part l2 and the end E8 of the part ll.

The chamber I4 is for receiving the gas to be tested and to this end the part II is provided with a, threaded nipple l9 having a hole 20 leading to the inside of the chamber It, through which gas from the source of gas to be tested GClaims. (c1. 1774511) 7 a standard gas which in this instance may be air. The disclosed apparatus will be described, for purposes of convenience, with reference to the testing of the purity of helium, that is;

, whether the gas in the chamber It is 100% pure helium or whether it contains some percentage of air. It will however be apparent, as the description of the invention proceeds that the apparatus is not limited to testing the purity of helium, but that the invention may be used for testing other gases and gas mixtures.

Disposed in the chamber i5 is a thermomotive device 23 in this instance comprising a bi-metallic strip 25 of horseshoe form mounted on a threaded stud 25 threaded in athreaded hole 26 in the partition it. 'If. desired a lock nut 25a. may be provided onthe stud 25. Developed flat the bimetallic strip,'26 appears as shown in Figure 4. The strip 26 proper is relatively narrow and is provided with enlarged ends 27, 28 having apertures 29. The aperture 29 in the end 21 is disposed about the stud 25 and this end may be held in position on the stud by nuts 30 (Figure 2) The other end, 28, of the bi-metallic strip 2 carries a, contact 3| having a shank 32 disposed through insulation 33 disposed between the shank maybe introduced into the chamber. Another threaded nipple 2 I, having a hole 22 communicating with the chamber It, is provided to permit the gas to be tested to displace any air or gas previously in the chamber. The nipple 2| may and the aperture 29 in the end 28.

The contact 3| is cooperable with a relatively stationary contact 34 provided in this instance by the end of a screw 35 cooperating with a threaded hole at in the stud 25 (Figure 2.) The screw 35 may be provided with any suitable means for adjusting it as for example a nut or a knurled collar 1-38 fastened thereto. The screw 35 may be held in adjusted position by a nut 39.

The bi-metallic strip 24 is provided with a heating coil so, the lower end of the coil (as viewed in Figure 1) being connected to the shank 32 of the contact 3! and the upper end, as viewed in Figure 1, being connected by a conductor 4! to the shell 62 of a lamp socket suitably mounted in this instance on the partition It. The shell 42 of the lamp socket and hencealso the upper end of the coil 60 is electrically connected by a conductor 43 to a. terminal plug 44 extending through a wall of the chamber l5 and supported and insulated therefrom by insulation 45. The terminal 44 may be connected by any suitable connection means to one terminal of a. battery 46 (Figure 3) or other suitable source of electric current. The other terminal of the battery is here shown as grounded. The bi-metallic strip 26 is so constructed that when heated the contact 3| tends to move away from the contact 34.

Disposed in the chamber I4 is a thermomotive device 41 in this instance comprising a bi-metallic strip 48 of horseshoe form mounted on the upper end of the stud 25. The bi-metallic strip 41 is similar to the bi-metallic strip 24, one end, 21a, being held in position on the stud 25by nuts 30a and the other end, 28a, carrying a contact 49 having a shank 50 and insulated from the strip by insulation The bi-metallic strip 48 is provided with a heating coil 62 the lower end of which is connected to the stud 25, and hence to the contact 34, and the upper end of which is connected by conductor means 53 to the casing part P2. The casing I 0 is grounded, and since as here shown the other terminal of the battery 46 is grounded, the upper end of the coil 52 is connected through ground to the grounded terminal of the battery.

The conductor means 53 is here shown as including a metallic insert 54 interposed therein, this insert fitting in and extending through a hole in the partition IS.

The shank 50 of the contact 49 is here shown as connected by a conductor 55 to a metallic insert 56 fitting in and extending through a hole in the partition IS, the other end of the insert serving as a lamp socket contact for engagement by the central contact 51 of a lamp bulb 58. Any other suitable electroresponsive means may be used in place of the lamp 58.

The contact 49 is adapted to cooperate with a relatively stationary contact '60 fastened, in this instance, in any suitable way to a flexible wall means portion of the chamber l4, in this instance a circular metallic diaphragm 6| the margin of which engages a shoulder 62 on the part II and is fastened thereto in any suitable way to form a gas-tight joint. The bi-mtallic strip 48 is so constructed that when heated the contact 49 tends to move away from the contact 60.

Means is provided to move the diaphragm 8! and hence the contact 60, to adjust the pressure between the contacts 49, 6|], this means as Here shown comprising a threaded boss 63 carried by and suitably fastened to the upper side of the diaphragm 6|, and a shaft 64 having a reduced threaded end 65 cooperating with the threaded boss 63. The shaft 64 is freely rotatable in a hole 66 in the upper wall '61 of the casing part I l, and is provided with a collar 68 adapted to engage the outer surface of the wall 61'. A spring 69 is provided for biasing the collar 68 and. hence the shaft 64 downwardly the spring 69 engaging at its upper end the inner surface of a transverse flange on a nut H screwed on a threaded cylindrical flange 12, on the wall 61, co-axial with the hole 66. The shaft 64 may be turned by a handle or knob 13 fastened to the shaft 64 by a ,set screw 14. The knob 13 is provided with a pointer 15 for cooperation with a temperature scale indicated by the legend, which may be marked on the adjacent surface of the upper wall 6'! of the casing part II, analogously to the scale shown in Figure 8, the indicia however being different as will more fully appear.

A wall of the casing part i2 may be provided with any suitable transparent window W through 4 has been calibrated to give a desired indication when the temperature of the ambient air and hence the temperature of the casing is a certain number of degrees. The position of the contact 60 is such that a certain amount of contractin flexure of the strip 48 is caused, and the position of the contact 34 is such that a certain amount of spreading flexure of the strip 24 is caused. Assuming how that the apparatus is connected to the battery 46, current will flow through the coils 40, 52 in series. When the strip 24 is sufficienty heated it will expand and interrupt the circuit through the coils at the contacts 3|, 34. As soon as the strip 24 cools the circuit will be completed again. Thls cyc le will be rapidly repeated. Meanwhile the current flow through the coil 52 will heat up the strip 48. The equilibrium temperature which the strip 48 would attain in a standard gas, such as air for example may be desired suitable temperature may be selected.

The temperature of equilibrium which the strip 48 reaches'when the chamber l4 contains the gas to be tested will depend on the heat conductivity of the gas which in turn depends on the purity of the gas. For example, pure helium has a certain heat conductivity which will result in dissipation of heat from the thermomotive means 41 at a certain rate and hence will result in the temperature of the thermomotive means reaching a certain value. By previous calibration this temperature is insuflicient to cause the contact 49 to be moved by the thermomotive means 41 out of engagement with the contact 60. Hence the circuit through the lamp 58 will remain completed. If however the gas in the chamber is say, helium and 5% air, the heat conductivity is less than for pure helium so that the rate of heat dissipation from the thermomotive means 4!v is less and the temperature it reaches will be higher. By previouscalibration this temperature'will cause the thermomotive means 41 to move the contact 49 out of engagement with the contact 60 and cause the lamp 58 to go out.-

Thus, if under the assumed conditions the helium in the chamber has a purity of, for example, 95% or less the lamp will go out and if the purity is in the range of above 95% to the lamp will remain lighted. It will of course be apparent that the apparatus may be so constructed and arranged and adjusted that the lamp 58 will not go out unless the purity of the helium is any other selected minimum percentage.

'In the foregoing it has been assumed that the apparatus has been calibrated and used at a given temperature of the casing III. In order to compensate for the effect on the thermomotive means 41 of a change in the temperature of the casing from the temperature at which the apparatus was calibrated the temperature of the casing I0 is determined and the knob 13 is turned until the pointer 15 indicates the casing temperature on the scale. This will .adjust the position of the engaging contacts 48, 60. For example, if the temperature of the casing is lower than the temperature of calibration, turning of the knob 13 to that lower temperature mark on the temperature scale will cause the contact 60 to tend to move away from the contact 49 a certain amount, the contact 49 however following the contact 60 and remaining in engagement therewith at a new definite position. The strip 48 is then contractively flexed a less amount than before so that a definite lower temperature of r the strip 88 will be required to cause separation of contacts 68, 68 thereby compensating for the lower casing'temperature. It will of course be apparent that if the temperature of the casing I8 is higher than that at which the apparatus. was calibrated, the knob '59 will be turned in the other direction to that higher temperature on the temperature scale, whereby the contact 88 is moved in a direction toward the contact 8-9 resulting in slightly further contractively flexing the strip 88 a definite amount. The amount that the contact 68 must be moved in a direction toward or away from the contact 4-9 to compensate for a departure in the casing temperature from the temperature at which the apparatus was calibrated with respect to percentage of purity of the gas may be readily determined and the temperature scale accordingly calibrated.

Referring now to the embodiment shown in t Figure 5, this apparatus comprises a casing I8b and casing parts IIb and I2!) similar. to the casing and casing parts l8, II, and I2 of Figure' 1, and similar chambers Mb, I5b are provided. Disposed in the chamber Mb for receiving the gas to be tested is a thermomotive device 88 comprising a bi-metallic strip 8| of horseshoe form mounted on a threadedstud 82 here shown as disposed through a hole in the insulating partition I61) and held by lock nuts 83. One end 212) of the strip 8| is enlarged and has an aperture through which passes a screw 84 threaded into the stud 82 and serving to hold the strip 8| in position. The other end 85 of the strip 8| carries a contact 86 which is in electrical contact with the stri 8|.

The thermomotive means 88 comprises a heating coil 81 for the strip 81, the lower end of this coil being connected by a conductor 88 to a metal insert 8-9 fitting in and extending through a hole in the. partition I6b, the other end of the insert 89 serving as a socket contact for cooperation with the central contact 98 of a ballast resistor 9| which'may be in the form of a so-called ballast tube or any other suitable means for maintaining a substantially constant current through the coil 81 even with a substantial variation in voltage of the source of electricity.

The thermomotive means 88 is so constructed and arranged that an increase of temperature will cause expansive movement of the strip 8|, that is, the contact 86 will be raised, the reverse being of course true for a decrease in temperature.

The ballast resistor 9| is here shown as in a socket the shell 92' of which is connected by a conductor 93 to a terminal of a switch 98 the other terminal of the switch being connected, by a conductor 95 extending through and insulated from the casing part I2b, or in any other suitable way, to a terminal of a battery 98 or other suitable source of electricity. The other terminal of the battery 96 is here shown as grounded.

The upper end of the coil 81 is here shown as connected by a conductor '91 to a metal insert 98 fitting in and extending through a hole in the partition 5b, the other end of the insert being connected by a conductor 81a to the lower terminal of a push button 99 (as viewed in Figure 5), this terminal being grounded to the casing by a conductor 91b. It will be evident that when the switch 84- is closed the coil 81 will be fed with current through the ballast resistor 9| from the battery 96.

The stud 82 and hence the strip 8| is electrically connected by a conductor I88 to a metal insert I8! fitting in and extending through the partition I8b, the other end of the insert serving as a socket contact for engagement bythe central contact I82 of a lamp bulb M3 in a socket shell I88 which is connected by a conductor I to the shell 82 and hence to the upper terminal of the switch 94 through the conductor 98. The other end of the insert IN is connected by a conductor I88, insulated from the shell I84, to the upper terminal of the push button 99.

Also disposed in the chamber I 8b is a thermomotive device comprising a bi-metallic strip II8 of horseshoe form, not provided with any heating coil but otherwise similar to the strip 8| but of diflerent length as will more fully appear. The strip II8 is mounted on a stud III in a manner similar to that in which the strip 8| is mounted on the stud 82. The stud III is here shown as threaded in a threaded hole in the upper wall 2 of the casing, a lock nut H3 being provided. The joint between the stud III and the wall 2 is preferably made gas-tight in any suitableway.

The thermomotive means or strip I I8 carries in electrical contact therewith at its free end a contact III and is so constructed and arranged that a decrease or temperature will cause expansive movement of the strip, that is, the contact I I4 will be lowered, the reverse being of course the case for an increase in temperature.

The-construction and arrangement is such that normally the contacts 88, Illare separated a certain distance (which may be adjusted by adjustment of either the stud 82 or I I) and so that the thermomotive device 88 may move the contact 86 into engagement with the contact I I4. When that occurs a circuit is completed for the lamp I83 from the battery 96, through the switch 84, through the conductor 83, shell 92, conductor I85, shell I84, lamp I83, insert I 8| conductor I88, strip 8|, strip H8, to ground (the strip II8 being grounded by its conductive mounting on the easing) back to the grounded terminal of the battery 98. It is of course apparent that the lamp I83 will be lit when thegas in the chamber MD is of a-certain minimum purity according to the selected adjustment and calibration of the appara us.

A wall of the casing part I2b may be provided with any suitable transparent window Wb through which the lamp I83 and other parts may be seen.

The push button 98 has a movable member I I5 adapted to -electrically connect the upper and lower terminals of the push button and it will be evident that when, so connected the contacts 86, I It will be by-passed, the insert I8I bein thereby grounded, so that the lamp I83 will light, if in operable condition. The push button 99 thus provides means for testing whether the lamp I83 is in operable condition.

The purpose of the thermomotiv means II8 is to compensate automatically for temperature-responsive movement of the thermomotive means 88 caused by a change in the temperature of the casing |8b from the temperature at which the apparatus was calibrated. If, for example, the temperature of the casing I8b is a certain number of degrees lower than the-temperature of caliof the gas is a predetermined selected minimum a temperature change of the casing I0b and the temperature change caused thereby in thethermomotive means. apparatus built embodying the invention a drop in temperature of the casing Iflb of one degree causes a drop in temperature eight-tenths of a degree in the heated thermomotive means 80. The deflection caused by a change in temperature of either the strip 8| or the strip H0 is proportional to the squareofthe length of the respective strip. Hence if, in this instance, the strip H0 is, made of a length equal to the square root of eight-tenths multiplied by the length of the strip 8|, the deflection of each strip 3|, I I 0, caused by a change in temperature of the casing will be the same and hence the desired compencating action Will be attained.

Referring now to the embodiment shown in Figures 6 through 8. There is here shown a casing I20, in this instance of oblong rectangular form, which is here shown as having a partition I2I although the partition may be dispensed with. In the casing I is disposed a thermomotive means I22 comprising a bi-metallic strip I23 of horseshoe form an end I24 of one arm of which is mounted on an end I25 of one arm of another bi-metallic strip I26, the ends I24, I25 being spaced by a spacer member I21 and held together by a screw and nut I28, the screw being disposed through the ends I24, I25 and spacer' I21. The end I29 of the other arm of the strip I26 is mounted on and held to any suitable bracket I30 by a screw and nut I 3I, the bracket being here shown as fastened to the bottom wall of the casing I20 by screws I32. I

The thermomotive means I22 includes a heat ing coil I33 the lower end of which is connected by a conductor I34 to one terminal of a ballast resistor I35 similar to the ballast resistor 9|, the other terminal of the ballast resistor I35 being connected by a conductor I36 to a conductor I31 one end of which is connected to a terminal of a switch I30, the other terminal of the switch being connected to a binding post I39. The upper terminal of the coil I33 is connected to the shank I M of a contact I40 carried by and insulated from the upper arm of the strip I23. The shank ml of the contact I 40 is also connected by a conductor I42 to a binding post I43. Thus when the switch I3'8 is closed, the coil I33 will be supplied with current from a source of current I44 connected to the binding posts I39, I43 as indicated in Figure '7.

The contact I40 is cooperable with a relatively stationary adjustable contact I45 which is carried by and insulated from a disc I46 which seals an opening of a flexible wall portion of the casing I20, this flexible wall portion being. in this instance a metallic bellows I41 the lower end of For example, in a particular casing I20.

is metallic, the conductors I34,

which is sealed by the disc I46. The upper end of the bellows I41 is fastened, in any suitable manner, gas-tight to the upper wall I48 of the The contact I45 is connected by a conductor I49 to one terminal of a lamp I50, the other terminal of which is connected to the conductor I31. It will be evident that engagement 'of the contacts I40, I45 will result in lighting of the lamp. I

The disc I46 is provided with a boss I5I having a threaded hole cooperable with the threaded end I52 of a shaft I53 which extends through a hole in the wall I48 and is rotatable in a bearing member I54 suitably fastened to or forming a. part of the wall I48. The shaft I53 has a collar I55 adapted to bear against the top of the bearing member I54. A spring I56,- disposed about the shaft I53, is provided, one end of the spring engaging the inside of the wall I48 and the other end engaging the boss I5I, so that the boss, and

, hence the contact I45, is biased downwardly to shaft I53 is rotatable by a knob I51 having a pointer I58 cooperable with a scale on the outer surfaceof the wall I40, the scale being designatod in Figure 8 with the legend "Purity scale." Desirably the cooperating threads on the shaft I53 and boss l5I are of fine pitch so that a relatively large amount of rotation of the knob I51 will produce a small movement of the contact 545. The scale of Figure 8 may be calibrated and marked in percent of purity of the gas to be tested in any desired range from purity to any desired lower percent purity within the selected desired range of the apparatus.

.Nipples I 90, 2Ic correspond to the nipples I9, 2| of Figure 1 and serve the same purpose. The upper wall I48 of the casing is here shown as provided with an aperture in which is mounted'a transparent member here shown as prismatic lens I59 which serves to transmit light from the lamp I50.

If a partition I2I is used, and if the partition I42, I49 may extend through insulating bushings I60 disposed through holes in the partition.

The operation of the embodiment shown in Figures 6 through 8 is as follows: The strip I26 is so constructed that a rise in temperature will cause the arms thereof to spread relatively to each other, that is, the end I25 will rise carrying the strip I23 with it. On the other hand the strip I23 is so constructed that a rise in temperature will cause the arms thereof to approach relatively to each other, so that the contact I40 will move downwardly toward the end I24 of the strip I23. Obviously, a decrease in temperature will have the opposite respective effects on the strips. It will therefore be evident that as far as the casing temperature is concerned, the effect of a change in casing temperature on the position of the contact I 40 will be nil, and any departure of the contact I40 from its normal position will depend on the temperature rise of the strip I23 caused by the coil I33 as influenced bythe heat conductivity or purity of the gas to be tested.

By having the arms of the strips I23, I26 directed in the same direction in combination with the construction and arrangement hereinbefore setforth, the contact I 40, considering the infiuence of casin temperature only. will remain in the same position and in general axial alinement with the contact I45, for a very wide range of casing temperatures. The contacts its, use are therefore adapted to come into substantially fiat engagement. It may be stated that for the full range of the apparatus, the movement ofthe contact I60 due to a rise in temperature caused by the coil I83 as influenced by the gas being tested is very small. The gas to betested is introduced into the casing I20 and the switch 133' is closed, so that the coil I38 is fed with current from the source Md. The ballast-resistor I35 keeps the current through the coil I33 substantially constant. Sufllcient time is permitted to elapse to permit the thermomotive means I22 to reach a substantially constant temperature and then the knob I! is turned to move the contact I45 toward the contact I40 until these contacts are in engagement which will be indicated by the lighting of the lamp I50. The

pointer I58 then indicates the percent purity of the gas being tested this purity being directly readable on the purity scale. It will of coursebe apparent that the embodiment of Figures 6 through 8 may be used to measure the purity of a gas through the entire range of purities from 100% to any desired lower percentage.

While the invention has been described with reference to embodiments for the testing ofhelium or similar gas having greater heat conductivity than air, it will be evident to those skilled in the artthat the invention maybe readily embodied in apparatus for use in test-- ing a gas the. heat conductivity of which is less than that of air or some other component of impurity.

From the foregoing it will be apparent to those skilled in the art that each of. the illustrated embodiments of the invention provides a new and improved means for testing gases and accordingly, each accomplishes the principal object oi! the invention. .On the other hand, it also will be obvious to those skilled in the art that the illustrated embodiments of the invention may be variously changed and modified, or features thereof, singly or collectively embodied in other combinations than those illustrated, without departing from the spirit of the invention, or sacrificing all of the advantages thereof, and that accordingly, the disclosure herein is illustrative only, and the invention is not limited thereto.

I claim:

1. Means for testing a gas, comprising: relatively movable contacts; means, responsive to the purity of the gas to be tested, constructed and arranged to move one of said contacts relatively to the other to a position dependent upon the purity of said gas; means for adiusting the other of said contacts to engage said one of said contacts at said position; means for indicating when said contacts come into such engagement; and means, including a scale, for indicating the purity of said gas by the position of said adjusting means with respect to said scale.

2. Means for testing a gas, comprising: relatively movable contacts; thermomotive means, responsive to the purity of the gas to be tested, constructed and arranged to move one of said contacts relatively to the other to a position dependent upon the purity of said gas; means for adjusting the other of said contacts to engage said one of said contacts at said position; means for indicating when said contacts come into such engagement; means, including a scale, for indicating the purity of said gas by the position of saidadjusting means with respect to said scale; and means 10 for compensating for the eflect of ambient temperature on said thermomotive'means.

3. Means for testing a gas, comprising: casing means providing a chamber for containing gas to be tested and comprising flexible wall means; relatively movable contacts; means, disposed in said chamber and responsive to the purity of the gas to be-tested, constructed and arranged to move one of said contacts relatively to the other; means whereby the other of said contacts is carried by said flexible wall means; and means, operatively related t said flexible wall means, constructed and arranged to move said flexible wall means to move said other of said contacts to adjust the position of at least said other of said contacts.

4. Means for testing a gas, comprising: a relatively stationary first contact; means, including a first U-shaped bi-metallic strip and a second contact mounted on a leg oi! said first strip, constructed and arranged to be responsive tothe purity of the gas to be tested and move said second contact relatively to said first contact along a given line; means, including said contacts, constructed and arranged to indicate whether said gas is above or below a selected purity; and means, including a second U-shaped bi-metallic strip one leg oil which is mounted on a relatively stationary support and the other leg of which is rigidly connected to the other leg of said first strip, constructed and arranged to compensate forthe effect, of ambient temperature on said purity responsive means and so that said path along which said second contact is movable by said purity responsive means remains substantially unaltered by the action of said second bimetallic strip.

5. Means for testing a gas, comprising: relatively movable contacts; means, including thermomotive means and heating means for heating said thermomotive means, constructed and arranged to respond to a change in thermal conductivity of said gas due to change in purity of said gas, to move one of said contacts relatively to the other to a position dependent upon the purity of said gas; said heating meanscomprising an electric heater for supplying heat to said thermomotive means at a selected rate dependent only on the electrical energy supplied to said electric heater; means for adjusting the other of said contacts to engage'said one of said contacts at said position; means for indicating when said contacts come into such engagement; and means, including a scale, for indicating the purity of said gas by the position of said adjusting means with respect to said scale.

6. Means for testing a gas, comprising: relatively movable contacts; means, including thermomotive means and heating means for said thermomotive means, constructed and arranged to respond to a change in thermal. conductivity of said gas due to change in purity of said gas, to move one of said contacts relatively to the other to a position dependent upon the purity of said gas; said heating means comprising an electric heater for supplying heat to said thermomotive meansat a selected rate dependent only on the electrical energy supplied to said electric heater; means for adjusting the other of said contacts to engage said one of said contacts at said position; means for indicating when said contacts come into such engagement; means, including a scale, for indicating-the purity of said gas by the position of said adjusting means with respect REFERENCES CITED The following references are of record in the file of this patent: 3

UNITED STATES PATENTS Name Number Date DiLegge May so, 1916 Number Number Name Date Willenborg Aug. 4, 1936 Shakespear, May 20, 1919 Peters Aug, 12 1924 Robertson Apr. 4, 1944 FOREIGN PATENTS Country Date Great Britain May 11, 1939 

